Glass resistant refractory and batch composition for producing such a refractory



Patented Jan. 23, 1934 UNITED STATES PATENT OFFICE .GLASS RESISTANTREFRACTORY AND BATCH COMPOSITION FOR PRODUCING SUCH A REFRACTORY Conn, acorporation No Drawing. Original of Delaware application February 21,

1930, Serial No. 430,459. Divided and this application April 16, 1932.Serial No. 605,785

s 6 Claims.

This application is a division of my copending application Ser. No.430,459, Patent No. 1,359,227, granted May 1'7, 1932 filed February 21,1930, and a continuation in part of my copending application Serial No.345,614, filed March 8, 1929.

The present invention relates to refractories resistant to the attack ofmolten glass such as glass feeder parts and other glass-contactingimplements, and particularly massive objects, such as tank blocks.Further, the invention relates to batch compositions, especially adaptedfor use in the manufacture of such refractories,

The general object of the present invention is to provide a novelrefractory of the above character possessing a fine-grained or vitreousstructure free from channels and voids, and of low porosity, such minutepores as are present being uniformly distributed. Such refractory alsopreferably consists of two phases only, microscopic aluminum silicatecrystals and a glassy matrix, such crystals being uniformly dispersed inthe glassy matrix, no crystalline silica being present. A further objectof the invention is to provide a novel glass resistant refractoryderived from grog and a binder of substantially the same refractoriness,but in which the interaction therebetween has been such as tosubstantially destroy ,the identity of the grog, thus providing a bodyof such uniform character that portions of the grog or binder are notseparately detached in use, but both are slowly worn away atsubstantially the same rate.

Such properties of glass contact refractories, such as tank blocks, arevery advantageous as a practical matter; they prevent localizedcorrosion or pitting such as occurs with ordinary tank blocks, resultingin early failure and shutdown of the tank for repairs; they also preventstoning or the dislodging of particles of large size which are entrainedin the glass, and remain therein forming defects or stones in thefinished glassware. By avoiding these and other defects which commonlyresult from the use of ordinary tank blocks, the manufacturer employingthe refractories of the invention is able to operate his tank for arelatively long period, and to produce glassware of better quality andmore economically than heretofore.

Another object of the invention is to provide a novel glass contactrefractory of such chemi cal composition that it is exceptionallyresistant to the corrosive action of molten glass. This object may beattained first by maintaining the total iron and alkali metal oxidecontent below a certain critical value, such as approximately 1.5%, andsecond by providing such a content of silica that a desirably largeamount of glassy matrix is formed which dilutes the above and otheroxides, substantially all of which are dissolved in the matrix. Thisresults in a matrix of high softening point, and correspondingly highresistance to glass attack. Preferably other imof only a small amount ofiron oxide and/or the character or degree of oxidation of the iron ofsuch oxide.

The above and other objects are attained to greater or less extent inthe use of novel refractories of the character, and produced by thenovel process and from the novel batches, described and claimed in mycopending application Serial No. 345,614.

A more specific object of the present invention is to provide arefractory which may be produced from cheaper materials than that of mylast named application, and which, though enriched by silicious materialto increase the amount of glass matrix, has a higher alumina content,and a higher content of aluminum silicate crystals.

Another object of the invention is to provide a novel batch compositionfrom which the novel refractory may be produced, and which in additionto being cheaper, as above stated, affords a wider margin of control infiring, particularly in the case of large objects such as tank blocks.It

is very difficult to fire such large massive objects at a temperaturesufllciently high to develop the above-described properties, especiallywhen the objects are shaped by dry-pressing. This results from therequirements of critical conditions, due to mechanical and chemicaldehydration, oxidation, and decomposition, which if not complied with,are apt to result in cracking, loss of shape, and other defects in thefinished objects. Such requirements are largely eliminated by the novelbatch composition of this invention, so that a relatively wide range ofcontrol in firing is obtained.

Two types of products and respective processes for preparing them willbe described as examples of the invention: first, the high silicaproduct described in my copending application Serial No. 345,61 l, andknown as the 901-D type; and second, the improvement thereon, known as90l-DB.

1. The 901 -D type For making one kind of dry-pressed product referredto herein as 901-13, I may employ a batch mixture consisting, in dry orcalcined equivalents, of about of washed Georgia Klondike white kaolin,which is mined near McIntyre, Wilkinson County, Georgia, 17% of groundpotters flint and 8% of an Arkansas clay, described as No. 23 on pages24 and 25 of Technologic Paper No. 144 of the U. S. Bureau of Standardsof January 28, 1920. This clay contains a considerable quantity, about55%, of exceedingly fine-grained silica and is characterized by lowshrinkage and by the fact that it does not overburn or bloat. Thecomposition of these three ingredients is indicated in thefollowingtable, which gives analyses of typical samples:

Georgia Klondike Arkanss Potters white k'io- 23 iii t (ignited) lin(ignited) l. 86 l. 14 0. 50 2. 59 0. 03 trace 1.02 0. l0

0. 95 N 20 0. 07 1. 22 Loss on ignition O. 20

These ingredients are mixed in the raw state. A grog is first preparedby mixing about 90 parts of the Georgia Klondike kaolin and 10 parts ofthe Arkansas clay No. 23, both ingredients being first groundexceedingly fine, suitably fine enough to pass through a standard U. S.No. 325 screen. The ground kaolin and the Arkansas clay are mixed asthoroughly as possible in the presence of water, by blunging, pugging orother approved methods, so as toproduce intimate association of theingredients. Since it is important that the iron content of the finalproduct be kept as low as possible, it is desirable that the materialsbe ground. in a rubber-lined mill and mixed in a porcelain-lined pugmill or other mixing apparatus having non-ferrous surfaces. The mixedmaterial is formed into pieces suitable for handling which are dried andare calcined at a temperature sufiicient to develop miscrosoopiccrystals, typically about 2850 F.

The calcined grog material is broken down to such fineness as to passthrough a standard U. S. No. 24 screen. A binding mixture is made fromabout 65 parts of the raw Georgia Klondike kaolin referred to above, 5parts of the Arkansas clay and 30 parts of potters flint. The Klondikekaolin and Arkansas clay are ground as before, to exceeding fineness,suitably sufficient to pass through a standard U. S. No. 325 screen, andthe potters flint may be ground to similar fineness or may be used in acondition, readily obtained commercially, in which practically allpasses through a No. 100 screen and all but about 7% passes through aNo. 300 screen. The ingredients are intimately mixed, as in the case ofthe grog material, and the mixing is preferably carried out by means ofa pug mill or other mixing apparatus having non-ferrous surfaces, inorder to prevent contamination of the material with iron, which isreadily abraded from iron mixing surfaces by the flint present in thematerial.

The final assembly for the blocks ismade by mixing about 17 parts of thegrog with about 20 parts of the binder, these proportions correspondingto about 46% of the grog and 54% of the binder. This mixture is formedinto the blocks or other shapes desired, which are preferablydry-pressed under a pressure of the order of 5 tons per square inch. Thematerial, when pressed, should contain about 6% to 12% of water. Theblocks or other articles are then dried and fired at a temperature ofabout 285i F.

Dry-pressing is considered to be essential in making this particularproduct, because the materials, before firing, have little or noplasticity or mechanical strength, either wet or dry. Therefore theblocks cannot be made by ordinary methods of soft mud molding or stiffmud molding or by pugging and extrusion.

It will be noted that in this particular product all of the addedsilica, in the form of flint, is incorporated in the binder and not inthe grog This gives a matrix which is materially higher in silica thanthe grog and increases the resistance of the product to solution,shrinkage, and spalling.

The silicious product described above is White in color, or nearly so,is dense, and has a bulk porosity less than 1%, and a total porosity ofless than 10%, the pores which are present being small, round andenclosed by dense walls. This product is further characterized by itshigh silica content in proportion to its effective life in contact withmoltenglass, by its low content of iron and alkali metal fluxes, and byan internal structure consisting of crystals which are uniform in sizeand very minute, typically about .02 mm. x .003 min. in size,distributed uniformly in a matrix constituting from 40% to 75% of themass and. composed wholly of amorphous glassy material which is low influxes and contains no crystalline silica. Its fusion point is above3100 F.

This product is also characterized by the presence within it of twophases only, typically mullite crystals and glass. This indicates thecomplete conversion of the starting materials, no part of which isdistinguisha...le in the final product. .i

Other high-temperature refractory products, heretofore proposed, oftencontain at least three phases, such as mullite, glass and quartz,mullite glass and crystobalite, mullite, corundurn and glass, etc.

The fine-grained and dense structure, and the microscopically small anduniformly distributed crystals. are considered to be the result of theextremely fine subdivision and intimate mixture of the startingmaterials and the develop-merit of th crystals in sit-u at a firingtemperature short of fusion. Under these circumstances the crystalscannot grow beyond microscopic size, as would happen if the ingredientswere not finely subdivided, or if the mixed material were heated tofusion.

I have determined by actual test of this silicious product in contactwith molten glass of an ordinary soda-lime type, that the solution ofthe refractory material in the glass does not materially distort theindex of refraction of the glass as other glass-engaging refractorieswill do, especially such as are highly aluminous. This avoids or reducesto a minimum the formation of .caused by the clays.

cords and streaks in the glass, which result from the solution of otherrefractories.

2. The 901-DB type Considering now the details of the 90l-DB type ofproduct embodying the invention, one example of the mineral compositionof the final assembly or batch composition which may suitably beemployed, is as follows:

Per cent Non-plastic kaolin 62.5 Plastic bond clay 30 Feldspar 7.5

The non-plastic kaolin may be of the character of Georgia Klondike whitekaolin, which is mined nearMcIntyre, Wilkinson County, Georgia, andwhich is a soft, white and non-plastic kaolin. The plastic bond clay maybe of the character of Georgia G1 clay, which is a plastic shrinkage ofthe plastic bond clay, and performs the further function of adding tothe silica content of the product and producing rigidity at high firingtemperatures and under high temperature conditions in service. Thiskaolin alone .does not become impermeable below 2800 F. and has onlyslight mechanical strength when raw. It has a very small firingshrinkage and a high softening point (i. e. Cone 34).

The G1 clay has a considerable degree of plasticity and performs thefunction in this mixture of still further adding to the silica contentand producing rigidity under heat, but is primarily used to give theinitial mixture sufficient temporary strength for molding and handling.The

.;-Gi clay also contributes to the final mechanical strength of theproduct. It toohas a high softening point (i. e. Cone 34) The feldsparlikewise adds to the silica content of the mixture and thus increasesthe 'famcunt of glassy matrix, but its primary purpose is to control therate and range of vitrification and the viscosity of the glass formedduring firing and the viscosity of the glass in the finished body, andto offset the volume changes or effects thereof The feldspar has a highviscosity which retards its inter-action with the surrounding claygrains, and it has a wide temperature range of vitrification and hencewidens the range of the vitrification of the mixture as a whole; ittends to prevent warpage or cracking during firing, and permitsrelatively rapid annealing of the blocks without injury thereto.

It should especially be remarked that the amount of feldspar which isadded tothe batch i: is relatively small and is selected according toits contents of iron and alkali metal oxides and according to likecontents of the clays or kaolins, so that when the batchis subjected tofabrication as hereinafter specified, or to a similar 1' ceramicprocess, the finished objects will contain described are. indicated inthe following table, which gives analyses of typical samples:

The process employed in fabricating the novel refractory materialembodying the present invention is or may be identical with thatdisclosed and claimed in my prior application, Ser. No. 430,459, andinvolves the preparation of a grog and of a binding mixture, and themixing of the grog and the binder in suitable proportions to form afinal assembly which is dry pressed or suitably molded into the finalshape desired.

The preparation of the grog One example of a batch mixture which maysuitably be employed for the grog in carrying out my invention is asfollows:

Parts Georgia Klondike kaolin 62.5 Georgia G1 clay 30 Feldspar 7.5

The three ingredients for the grog first are ground exceedingly fine,suitably fine enough to pass through a standard U. S. No. 325 screen.

To indicate the degree of fineness to which I prefer to sub-dividestarting materials, it may be noted that the standard No. 325 screenreferred to above is identified by the U. S. Bureau of Standards ashaving a sieve opening of .044 mm. (.0017), a wire diameter of .036 mm.(.0014"), with a tolerance in average opening of plus or minus 8%, atolerance in wire diameter of minus 15 to plus 35%, and a tolerance inmaximum opening of See Bureau of Standards Specifications for Sieves, U.S. Standard Sieve Series.

The ground kaolin, G1 clay, and feldspar are mixed as thoroughly aspossible in the presence of water, by blunging, pugging, or otherapproved methods, so as to produce intimate association of theingredients. Since it is important that the iron content of the finalproduct be kept as low as possible, it is desirable that the materialsbe ground in a rubber-lined mill and mixed in a porcelain-lined pug millor other mixing apparatus having non-ferrous surfaces. The mixedmaterial is formed into pieces suitable for handling, which then aredried and are calcined at a temperature suflicient to developmicroscopic crystals, typically at about 2350 F.

The calcined grog material is broken down to such fineness as may bedesired, suitably fine enough to pass through a standard U. S. No. 24

employed.

Microscopic examination of grog produced in the above manner under highpower (i. e. 5203:)

screen, although coarser or finer grog may be shows that it is composedof extremely minute crystals of aluminum silicate uniformly distributedin a glassy matrix. In some cases, the glassy matrix may contain a fewresidual quartz splinters, which, however, are dissolved when the grogsubjected to high temperature, as ,describedhere- The grog is veryhomogeneous micro-..

inafter. scopically and macroscopically, is well vitrified, and hence oflow porosity and high density.

The use of the feldspar in preparing the grog makes it possible toobtainthe above described transformation at a lower temperature and withgreater ease than if the clayswerefired alone. This effects economy infuel consumption. Moreover, the derivative of the feldspar, assists inthe union of the grog with the binder, without hoW- ever impairing thehomogeneous character of. the grog and the finished product.

Preparation of the binder The binding mixture may consist of the samebatch composition as the grog, the proportions of the ingredientsbeingpreferably the same as in the grog mixture, although theseproportions may be varied somewhat if desired. The Klondike V kaolin andthe G1 clay are ground as before, to exceeding fineness, suitably topassthrough a standard U. S. No. 325 screen, and the feldspar likewisemay be ground to similar fineness, or may be used in the fine conditionin which it is obtained commercially, both for the grog and for thebinding mixture.

The ingredients are intimately mixed, as in the case of the grogmaterial, and the mixing preferably is carried out by means of a pugmill or other mixing apparatus, having non-ferrous surfaces, in order toprevent contamination of the material with iron, which is readily.abraded from iron mixing surfaces by the hard particlesof theingredients.

Preparation and shaping of the final assembly The final assembly for theblocks or other ob- ,jects preferably is made by mixing parts of thegrog with 35 parts of the binder. This mixture is formed into theblocks, or other shapes, preferably by dry pressing under a pressure ofthe order of 1 to 2 tons per square inch. The material, when pressed,should contain about 6% to 12% of Water.

The blocks or other articles are then dried and fired to completevitrification and partial or incipient fusion but without generalfusion. They may, for example, be fired to a temperature of about 2850F.

Dry pressing is preferably employed in making refractory productsaccording to the present invention, because the materials as assembled,be fore firing, have little or no plasticity or mechanical strength,either wet or dry. Therefore, the blocks cannot be made by ordinarymethods, of

soft mud molding, or stiff mud molding or by The products of the presentvinventionare white. in color or nearly so, are. dense and are of lowporosity. The pores which arepresent are small,

r und a e c edbyd e w lls; ds ep nir.

types of refractories.-

is introduced into a final assembly andagain formly distributedthroughout the. mass... Thenhave a bulk specificgravity of as high as2.448,;-

true specific gravity 2.72,.porosityby absorption of 8% to 0% (theWeight of water absorbed into the open pores in terms of the driedweight) and an. apparent porosity of less than 8% (the weight of thewaterv absorbedin the open pores in terms of theexterior volume).Certain special duty refractories may have a higherporosity (say 15depending upon the requirements of practice.

Refractories embodying the invention are further distinguished by theextensive reaction; between grog and binder which is such that the.

identity of the grog is substantially lost and the grog and bindercannot be separated mechanically; or'by the corrosive action of themolten,

glass thereon. Consequently, such refractories do not throw stones. Thishas been verified by actualcommercial use of tank blocks embodying theinvention, the glass produced being remarkably free from stones andother defects commonly caused by ordinary tank blocks, dueto theseparation of the grog, or large pieces ofv material, from the blocks.

The novel material of the invention is further characterized by aninternal or petrographicstructure consisting of microscopic aluminumsilicate (mullite) crystals Which'are substantial1yuniform in size, anda glassy matrix, in which such crystals are uniformly distributed, boththe body as a whole andthe glassy matrix contain ing no free. orcrystalline silica such as quartz.

In some samplesof the refractory the maximum length of the mullitecrystals is about 0.030 mm. and the maximum widthis about .006 mm. thecrystals occurring in well developed, elongated, and prismatic form.

Because of the small size andrelatively uniform distributionof thecrystals intheglassy matrix, and the interlocking of the crystals, it

is examination the exact amounts of crystals and glass respectivelywhich are presentin the body.

In fact, there is no known method by which-the relative amounts ofcrystals and glass in a refractory like that embodying the invention canbe ascertained exactly. The crystals are so intimately associated withthe-glass that it is impossible to separate them by chemical ormechanical methods, asis the case with certain other impossible todetermine by a microscopic However, it has been reliably estimated thatthe body consists of beglass by volume. Therefore, the crystalsconstitute between about 37% and about. 57%, and

weight- Such petrographic structure has the advantage of, offeringuniform resistance to the attack of the molten glass, or in other words,a refractory of such structure wears away very evenly and veryuniformly, no particles, or crystals ever appearing in the finishedglassware as .stones or specks, such-as are produced from refractoriescontaining largecrystals of mullite or corundum. The crystals inapplicants refractory are so small and so intimately associated witheach other and with the binder that they are dissolved in 'tween 33% and50% crystals, and 67% to 50% 1301:: the glass between 63% and 43% of thebody by the glass before they are detached from thebody,

or if detached, dissolve in the glass long before such glass isdelivered from-the tank.

The-refractory product ,of the-present inven- 1 tion. is remarkablydistinguished from prior refractories with respect to its low totalcontent ofv iron,.-alkalie, and alkaline oxides or fluxes,

especially in view of the actual addition of feldspar containing anappreciable amount of alkali metal oxide. This may be due to the firingof the batch to high temperature, the ingredients being so finelydivided as to permit some of the alkali metal oxide to volatilize fromthe body.

The composition of a typical finished object prepared according to theexample given above is indicated in the following table:

Ordinary commercial tank block has a total content of 2.75% of iron andalkali metal oxides,

whereas the above-described product of the presbatch, and variations inthe finished body may ent invention has a total content of only 1.44%

'of iron, alkalie, and alkali metal oxides.

It is preferred to maintain the total content 'of iron and alkali metaloxides (sodium and potassium oxides) below approximately 1.5%

because it has been found by experiments and tests that when thisapproximate value is exceeded, there is a surprisingly great decrease inthe resistance of the refractory to glass attack. For similar reasons,it is preferred that the total iron, alkali, and alkaline earth metaloxides not exceed approximately 1.5%, although in some cases this totalmay be as high as approximately 2% without appreciably afiecting theresistance of the material to glass attack. In fact, the

presence of small amounts of the alkali and alkaline earth metal oxidesin the raw starting materials, (corresponding to the above limits ofcomposition of the end product), is advantageous in that the firing ofthe finished objects is assisted thereby in somewhat the same manner asby the feldspar which is actually added to the batch.

But, in any event, it has been found desirable to maintain the contentof iron oxide as low as possible in order that glass of good color (thatis, free from the discoloring effect of iron oxide) will be produced. Infact, refractories embodying the invention have been produced frommaterials of the above character containing as low as .45% iron oxide.It is impractical to attempt to lower the iron oxide content belowapproximately 45% because that necessitates the use of startingmaterials which are too expensive for the economical and profitablemanufacture of the glass contact refractories contemplated by thisinvention, in large quantities.

In order-to insure that the novel refractory of the invention willcontain between approximate- 1y 50% and 67% glassy matrix by volume, orbetween approximately 43% and 63 by weight, it is preferred to increasethe silica content introduced by the clay or clay mixture, by theaddition of the proper amount, and the proper kind of feldspar,consistent with high fusion point. This is desirable because as theamount of matrix is thus increased, its silica content is increased andthe percentage of impurities therein is decreased, this yielding amatrix of high softening point, and having a 10W rate of solution in themolten glass, provided of course that the percentages of impurities inthe body as a whole parts by Weight of homogeneous granules of grog arekept within the limits above specified. Ordinarily my novel productswill contain from approximately 1.5 to 5% more silica than the bestgrade kaolin refractory, and therefore will have a higher content ofglassy matrix and at the same time a sufficiently high fusion point foruse in molten glass. The batch ingredients are so selected as to yield atotal of approximately 97% alumina and silica in the final product.

Tank blocks embodying the invention have given excellent service both inthe manufacture of soda-lime glass and glasses more severe in theiraction on refractories such as boro-silicate glass. In such service, theblocks wear away very slowly and quite evenly, and the glass produced isof superior quality to that previously melted in tanks, being free fromstones and other defects, and having exceptionally good color.

The resistance of my novel material to erosion is further evidenced bythe unusually long life of orifice rings for glass feeders composedthereof.

It will be understood that substitutions in the be resorted to withoutdeparting from the scope of the claims.

I claim:

1. A batch for a ceramic refractory resistant to molten glass.comprising approximately 65 185 containing derivatives of approximately92.5% of a mixture of Georgia Klondike kaolin and Georgia 2850 11, andhaving substantially the same physical and chemical characteristics asthe refractory to be produced, and approximately 35 parts by weight rawbinding material composed of approximately 92.5% of a mixture of GeorgiaKlondike kaolin and Georgia G1 clay and approximately 7.5% Bedfordfeldspar, both the mixture and the feldspar being substantially the sameas the mixture and feldspar from which the grog is derived.

2. A batch for a ceramic refractory resistant to molten glass,comprising not less than 65 parts by weight of homogeneous granules ofgrog containing derivatives of approximately 92.5% of a mixture ofGeorgia Klondike kaolin and Georgia G1 clay and approximately 7.5%Bedford feldspar fired to a temperature of approximately 2850 F. andhaving substantially the same physical and chemical characteristics asthe refractory to be produced, and not more than 35 parts by weight ofraw binding material composed of approximately 92.5% of a mixture ofGeorgia Klondike kaolin and Georgia G1 clay and approximately 7.5%Bedford feldspar, the composition of the binding material beingsubstantially the same as the composition of the mixture from which thegrog is derived.

3. A dense ceramic tank block substantially impermeable and resistant tomolten glass, consisting wholly of a glassy matrix and microscopiccrystals of mullite uniformly distributed there-. through, said glassymatrix containing no crystalline silica, said block containingapproximately 44% alumina and approximately 53.5% silica, the silicacontent however being in excess of the silica content of dehydratedkaolin (which contains approximately 52% silica), said block containinga total of iron and alkali metal oxids (Fezoz, Nazo and K20) notexceeding 1.5%, whereby the said block is highly resistant to thecorrosive action of molten glass and does not 5 LLLl substantiallydistort or discolor the glass, alkaline .earth oxids (MgO and CaO) andtitanium and other oxids being present only in the small proportions inwhich they occur as impurities in the clay and other materials fromwhich the block is made, said block containing not less than 33% .normore than 50% of said microscopic mullite crystals and between 67% and50% of said glassy matrix whereby said matrix has a low content offluxes and is high in silica content and therefore highly resistant tosolution in molten glass.

4. A dense ceramic tank block substantially impermeable to and resistantto molten glass, consisting wholly of a glassy matrix and micro- .scopiccrystals of mullite uniformly distributed therethrough, said glassymatrix containing no crystalline silica, said block containing approxi--:mately 44% alumina and approximately 53.5%

silica, the silica content, however, being in excess of the silicacontent of dehydrated kaolin (which contains approximately 52% silica)said block containing a total of iron and alkali metal oxids (F8203,NazO and K20) not exceeding 1.5% and --a total of said iron and alkalimetal oxids and alkaline earth oxids (MgO and CaO) not exceedingapproximately 2%, and being present with titanium and other oxids onlyin the small pro- ;portions in which they occur as impurities in theclay and other materials from which the block is made, said blockcontaining not less than 33% nor more than 50% of said microscopicmullite crystals and between 67% and 50% of said glassy matrix, wherebysaid matrixhas a low content of fluxes and is high in silica content,and therefore highly resistant to solution of molten glass.

.5. A dense ceramic tank block substantially impermeable to andresistant to molten glass consistingwholly of a glassy matrix andmicroscopic crystals of mullite uniformly distributed therethrough, saidcrystals being well developed, elongated and of prismatic form, themaximum size of which is in the order of approximately 0.03 mm. x 0.006mm, said glassy matrix containing no crystalline silica, said blockcontaining approximately, 44% alumina and approximately 53.5% silica,the silica content however beingin excess or" the silica content ofdehydrated kaolin (which contains approximately 52% silica), said blockcontaining a total of iron and alkali metal oxids (FezOa, NazO and K20)not exceeding 1.5%, whereby the said block is highly resistant to:thecorrosive action of molten glass and does not substantially distort ordiscolor theglass, alkaline earth oxids (MgO and CaO) and titanium andotheroxids being present only in the small-proportions in which theyoccur as impurities in the clay and other materials from which the blockis made, said block containing not less than"33:%, nor more than 50%, ofsaid microscopic mullite crystals and between 67% and 50% of said glassymatrix, whereby said matrix has a low content of fluxes and is high insilica content and-therefore highly resistant to solution in moltenglass.

6. A dense ceramic tank block substantially impermeable and resistant tomolten, glass, consisting Wholly of a glassy matrix and microscopiccrystals of mullite uniformly distributedtherethrough, said glassymatrix containing no crystalline silica, said block containing notlessthan 1 00 approximately 35% alumina and not more than approximately62% silica, the silica contentihowever being in excess of thesilica-content of 'dehydrated kaolin (which contains approximately 52%silica) said block containing a total of iron 195 and alkali metal oxids(F6203, NazO and, K20) not exceeding 1 whereby thesaid block-is highlyresistant to the corrosive action of molten glass and does notsubstantially distortordiscolor the glass, alkaline earth-oxids (MgO-and110 CaO) and-titanium and other oxids being-present only in the smallproportions in-which they occur as impurities in the clay andothenmaterials from which the block is made, saidiblock containing from40% to of said glassyzma- 115 trix whereby said matrix has a low contentof fluxes and is high in silica content and therefore highly resistantto solution in molten glass.

PAUL G. WILLETTS.

